some of it is annoying yes, but for some I think it comes from a good place. They usually end up plugging a charity or two, and/or just using their celebrity soap box to raise awareness. We don't always know if or how much they are paid to do the interviews or programmes about their illness, and if they announce they are doing it for free then people object to that as well.
In contrast, I have far less respect for these "celebrity" families that are paid money to allow a camera crew into their homes e.g. Tyson Fury, Stacey Solomon etc. They haven't actually suffered anything that the rest of us haven't. But complain about how tiring it is working and doing the school run, and how they're depressed or battling some addiction. The irony is that millions of others in the UK have to put up with the same or worse, whilst not sitting in a £1.3M house